Precise Evaluation of the Cochlear Duct Length by Flat-panel Volume Computed Tomography (fpVCT)—Implication of Secondary Reconstructions

Abstract

Hypothesis: Flat-panel volume computed tomography (fpVCT) and secondary reconstruction allow for more accurate measurements of two-turn length (2TL), cochlear duct length (CDL), and angular length (AL). Background: Cochlear geometry is a controversially debated topic. In the meantime, there are many different studies partly reporting highly divergent values. Our aim is to discuss the differences and to propose a radiological possibility to improve cochlear measurements using 3D-curved multiplanar reconstruction and fpVCT. Methods: Performing different image modalities and settings, we tried to find a clinically usable option that allows for a high degree of accuracy. Therefore, we tested them against reference values of high-definition micro-computed tomography. Results: Comparison of 99 μm slice thickness secondary reconstruction of fpVCT and reference showed no significant differences for 2TL and CDL (p ≥ 0.05). Accordingly, ICC (intraclass correlation) values were excellent (ICC ≥ 0.75; lower limit of confidence interval [CI] ≥ 0.75; Cronbach's alpha [α] ≥ 0.9). Evaluating AL, there was a significant difference (difference: −17.27°; p = 0.002). The lower limit of the CI of the ICC was unacceptable (ICC = 0.944; lower limit of CI = 0.248; α = 0.990). Regarding the Bland-Altman plots, there were no clinically unacceptable errors, but a systematic underestimation of AL. Conclusion: Secondary reconstruction is a suitable tool for producing reliable data that allow the accurate measurement of 2TL and CDL. The option of generating these reconstructions from raw data limits the need for higher radiation doses. Nevertheless, there is an underestimation of AL using secondary reconstructions.

Implementation of secondary reconstructions of flat-panel volume computed tomography (fpVCT) and otological planning software for anatomically based cochlear implantation

“…Indeed, the use of fpVCT SECO in this study, revealed the above mentioned advantages, in addition to the obvious reduced metal artifacts [20,21]. The demonstrated results are in accordance with recently published data on measurements of CDL OC using fpVCT SECO in temporal bone specimen [24] using multi-planar reconstructions [38]. This might be well explained by the higher resolution of fpVCT SECO with a slice thickness of 99 µm in comparison to the other image qualities.…”

“…Due to the larger artifacts of the electrodes in MSCT, the points marking the lateral wall might be set more laterally than in the true anatomical set, responsible for the discrepancies in different parameters. Therefore, clinical analyses using fpVCT SECO probably represents the true length, since assessments with micro-fpVCT and micro-CT in temporal bone specimens have shown no significant different CDL at the lateral wall values up to over 40 mm [13,24], which corresponds to a CDL OC of about 35 mm. In line with this, in other studies, shorter CDL values have been reported in clinical modalities [8,13].…”

“…Further advantages of fpVCT are portability-enabling the intraoperative use and reduced metallic artifacts, allowing postoperative position analyses, and more accurate frequency mapping of the electrode contacts [20][21][22]. As reported by Pearl et al fpVCT offers the possibility to enhance image quality by secondary reconstructions (fpVCT SECO ) [23], which can reach the same accuracy in measuring the CDL compared to experimental Micro-CT [24].…”

Cochlear Duct Length Measurements in Computed Tomography and Magnetic Resonance Imaging Using Newly Developed Techniques

“…In addition, the sample size might be too small, but previous studies investigating the CDL have shown that a number of 10 is adequate to obtain sufficient statistical power. 7 , 41 , 42 …”

“…In clinical practice, multislice computed tomography (CT) 3 , 4 and cone beam CT 5 , 6 are most commonly used. Experimental imaging systems such as micro-CT 5 , 7 and synchrotron imaging 8 have been shown to be more precise, but they are not applicable for use in humans due to their higher radiation exposure and the need for small imaging volumes.…”